The present invention relates to a method for producing a laminated sheath which is used, for example, in producing a laminated sheath cable composed of a laminated tape made of metal or the like using adhesives and a sheath which is made of plastics or the like which covers the tape in such a way as to form a lamination thereon.
In general, a laminated sheath has a characteristic feature of being advantageously moisture-proof as well as having a superior mechanical strength. Thus, laminated sheaths have been used in various kinds of products such as electric wire cables, pipes for conveying heated fluids, and waveguide tubes having an elliptic cross-section. An example of such a laminated sheath, which is commonly available at present is shown in FIG. 1. In FIG. 1, reference numeral 2 denotes an insulated wire core for an electric power cable, a communication cable or the like which is hereinafter simply referred to as a "core". The core 2 has a conductor around which an insulating material is provided covering the conductor or an assembly of a plurality of such conductors provided with insulating materials. If necessary, a metal shield, binder or the like may be provided on the core 2.
Around the core 2 is provided a laminated tape 5 which is formed by a metal tape 3 (for example, aluminum tape) with adhesives 4 (for example, ethylene copolymer) on one or both sides thereof with the laminated tape 5 partially overlapping itself to form a tube shape so as to cover the core 2. The overlapping portion 6 is heated so the ends of the tape which are in contact with each other are bonded to each other. Around the tape 5 which is thus laminated is provided a sheath 7 made of plastics such as polyethylene (PE), polyvinyl chloride (PVC) or the like covering the laminated tape 5. Thus, the sheath 7 is adhered to the laminated tape 5 so as to form a laminated sheath cable 1.
As to the technique for producing such a laminated sheath cable, there have conventionally been known the following methods:
A first method is illustrated in FIG. 2. This method is most commonly used.
In FIG. 2, the core 2 is supplied in a continuous manner from a supply stand 8 while at the same time a tape 5 to be laminated is longitudinally supplied around the outer periphery of the core 2 from a supply stand 9. The core 2 and the tape 5 then pass to a machine 10 which imparts a desired shape. The machine 10 operates to provide the laminated tape 5 in the form of a tube around the core 2 to thus cover the core 2. Usually, the machine 10 is constructed of a plurality of rolls or dies 11 for imparting the desired shape. The core 2 is covered with a tube composed of the laminated tape 5 as mentioned above. After initially covering the core, the core 2 and the tape 5 are preheated by a preheater 12 thereby subjecting the laminated tape 5 at the overlapping portion 6 (FIG. 1) to an adhering treatment. Immediately thereafter, the sheath 7 is extruded from an extruder 14 and applied over the laminated tape 5 around the core 2 covering the laminated tape 5 during the vacuumizing operation of the vacuum device 13. The laminated cable 15 thus produced is cooled while passing through a water-cooling trough 16 and taken up by a take-up device (not shown).
However, as can be seen from the sheath structure shown in FIG. 3, a problem may occur at the overlapping portion 6 of the tape in a laminated sheath cable which is produced in accordance with this method. That is, a gap is formed at a step portion 17 between the sheath 7 and the overlapping portion of the laminated tape 5. In addition, a step 18 is formed at the outer side of the sheath 7. Due to this, there is a drawback in that splitting may occur at these portions if a shock force is applied to the cable.
The laminated tape 5 which is used in this method is a coiled article which is separately produced. As a method for the production thereof, for example, it has been most common to provide adhesives on an aluminum tape using an extruder or a calender roll. In order to provide a thin layer of adhesives in a uniform thickness without damaging the covering layer of adhesives yet while enhancing the bonding strength between the aluminum tape and the adhesive, large and expensive manufacturing equipment is needed. As a result, there is the drawback in that the laminated tape is expensive.
Concerning the product quality, various drawbacks with this arrangement are also present. Specifically, during storage, blocking may occur between the laminated tapes. In addition, the surface conditions of the aluminum tape or adhesives can vary thereby lowering the bonding efficiency. Furthermore, the type of the adhesives used must be strictly chosen in order to avoid the above-mentioned problems.
A second method as illustrated in FIG. 4 may be used to overcome the drawbacks which are attendant with the first method. Reference numerals employed in FIG. 4 which are the same as those employed in FIG. 2 denote the same components. In FIG. 4, the tape which is supplied from the supply stand 9 is not laminated tape but instead is metal tape, for example, aluminum tape 19, which is formed into a tube shape around the core 2 by the machine 10. After that, the surface of the tube of aluminum tape 19 is covered with a resin 21 (adhesive) for the lamination process using an extruder 22. At the same time, the plastic sheath 7 is formed by extrusion to cover the resin 21. Reference numeral 20 denotes an extruder for the sheath which is supplied to the extrusion die portion. Other than the above-mentioned components, the apparatus of the second method does not differ from that of the first method.
In accordance with this method, it is not necessary to separately produce the laminated tape. However, other problems may arise. That is, when the aluminum tape 19 is shaped with the die 11 of the machine 10, the surface of the tape can be easily damaged inasmuch as the tape is not covered with the adhesives. Small pieces, cuttings or chips of aluminum produced when the surface of the tape is damaged can increase the resistance of the forming die. Finally, the aluminum tape 19 may be torn in half. This is a defect which should not be allowed to occur in a practical production process.
Considered from a viewpoint of quality, there are not only the same defects as in the first method in that a gap is formed at the step 24 of the overlapping portion 23 of the aluminum tape 19 as seen in the sheath structure which is shown in FIG. 5 thereby causing a weakness in the structure, but also other defects, for example, the weakening of the waterproof sealing of the laminated sheath and the mechanical strength of the cable may occur. Furthermore, a damaged surface of the aluminum tape may corrode by the action of water due to the fact that the overlapping of the aluminum tape 19 may not be in close contact with each other. Due to these problems, the reliability of the cable over a long period may be significantly lowered using this method. In FIG. 5, reference numeral 7 denotes a sheath and reference numeral 21 denotes adhesives.
In order to solve the problems which arise with the use of the second method, the first and second methods can be combined. In other words, before the aluminum tape is supplied and shaped, the adhesive such as resin used for laminating or the like is provided on one side of the tape portions to be overlapped using a calender roll or the like in such a manner as to cover the one side. Then immediately, the steps of shaping the tape and extruding the sheath are carried out in the same way as indicated in FIG. 2.
This combination of the first and second methods is disadvantageous in that the method is complex to implement, especially for the step of covering the surface of the aluminum tape with the adhesive, in the same manner as already stated in connection with the first method. Thus, it can be easily understood that the overall manufacturing efficiency is low as a whole if the above-mentioned covering step is carried out in conjunction with the step for extruding the sheath. In addition, the quality problem which is due to the step at the overlapping portions of the laminated tape as mentioned above has yet to be solved.
Nonetheless, the second method is very advantageous in comparison with the first method in that long cables can be produced in a continuous manner inasmuch as a metal tape welder, such as a cold welder which is conventionally available, may be employed without modificaion in connecting the tape. Thus, the second method is most suitable in applications requiring continuity at the connecting portion of an external conductor such as a laminated sheath coaxial cable for use with CATV. In addition, in view of the fact that the laminated tape can be produced simultaneously with the sheath covering making use of a unit member of aluminum tape, it goes without saying that the second method is advantageous in terms of production cost in comparison with the first method from a viewpoint of energy requirements.